BIOL 1090 Chapter Notes -Integral Membrane Protein, Electrochemical Gradient, Endoplasmic Reticulum

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12 Feb 2013
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What type of protein is represented by:
A. Peripheral Membrane Protein
B. Peripheral Membrane Protein
C. Integral Membrane Protein
2. Would you expect the following regions to
be hydrophobic or hydrophilic? Why?
D. Hydrophobic (highly) interacts with fatty acyl chains
E. Hydrophilic (highly) interacts with aqueous ECM
3. What is a polarized cell?
Spatial organization of organelles
A cell containing distinct plasma membrane domains (i.e, apical, lateral, basal)
Composed of distinct proteins giving rise to distinct properties (i.e directional transport
of molecules or adhesion)
4. Is a nerve cell a polarized cell? Explain.
Yes different regions of the plasma membrane contain distinct proteins. Some regions
(Dendrites) contain proteins involved in receiving incoming signals. Other regions
(terminal knobs) contain proteins involved in transmitting signals to target cells. Other
regions (axon) conduct impulses away from the cell body.
5. Define active transport.
Transport of a substance against its gradient The process by which a substance binds
to a transmembrane protein to allow the movement of a substance against its
electrochemical gradient using energy.
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The Dopamine Transporter
The dopamine transporter is responsible for the reuptake of dopamine from the
synapse.
It clears the dopamine from the synapse by pumping it back into the neuron (using
active transport)
Dopamine transporter is an integral membrane protein with 12 Transmembrane
domains
- The Dopamine is moved into the cell against its concentration gradient thus energy is
required to move it across
Reuptake of dopamine by the dopamine transporter is active transport
The DAT is a contransporter
-couples active transport to an existing ion gradient .
The Na/K-ATPase establishes a gradient of Na ions
-the concentration of NA is higher outside the cell than inside the cell
the Na gradient is stored energy
-The gradient is needed to transport the dopamine
The DAT uses this energy to move 2NA ions down its gradient and 1
dopamine molecule against its gradient it also moves 1 Cl ion into the cell
Since the NA and dopamine are moving in the same direction (into the cell)
this is called symport.
3 Sodiums get in the door, it turns the sodium gets kicked out the other side
and at the same time 2 potassiums get in . Phosphate that is released from
the ATP is bound to an amino acid and the potassium kicks off the phosphate
from the amino acid and it causes a change in the pump. Ultimately you end
up with a lot of sodium on the outside and a lot less potassium on the inside
even though you have a different charge you have a much different charge
on the outside than on the inside so the job of the pump is to create an
electrochemical gradient that can be used to power the symport of the DAT
Dopamine and the Pump
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High Sodium on the outside, Low sodium within and Dopamine are outside
and 2 sodiums go in and bring in a dopamine with them. This occurs across
the synapse. Carries the dopamine with them through the dopamine
transporter into the dopaminergic neuron using an electrochemical gradient
formed by the Na/K ATPase
Drugs and DAT
Some drugs act by competing for the DAT
-Amphetamine, Meth, Ecstasy, mephedrone, resemble structure of dopamine
and use the DAT To enter the cell
-They compete with dopamine therefore less dopamine enters the cell
-once inside the cell they also reverse the action of the DAT
-The DAT moves dopamine into the synapse
-blocks transporter therefore NO REUPTAKE OF DOPAMINE FROM THE
SYNAPSE
-The Dopamine transporter is perisynaptic (located beside the synapse). If
plasma membrane are fluid and dynamic how is this specific localization
maintained.
________
Where is DAT synthesized?
-It is synthesized at the Rough endoplasmic reticulum by membrane bound
ribosomes.
What is a signal sequence?
A stretch of amino acids at the n terminus which is recognized and bound by
the SRP. The signal sequence targets a protein for cotranslational import. If
a protein does not contain a signal sequence it will continue translation on a
free ribosome in the cytoplasm.
Does the nascent dopamine transporter contain a signal sequence?
Yes Because the DAT is an integral membrane protein on the plasma
membrane, it must be targeted to the lumen of the ER first.
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